This invention relates to box horn antennas in which the feed waveguide has smaller cross-sectional dimensions than the box horn for inducing higher order modes for linearizing the aperture distribution of the box horn antenna to achieve higher gain.
There are many applications for which antennas having shaped radiation patterns are desirable. For example, shaped beam antennas for communications applications use offset reflectors and multiple element array feeds. For microwave applications, the array feed elements are often in the form of waveguide horns. The gain of a shaped beam reflector antenna is dependent upon the radiation properties of the individual feed horn antenna, which in turn depends upon the illumination of the aperture of the horn.
Those skilled in the antenna arts know that the transmitting and receiving characteristics of an antenna are reciprocal functions. That is, the gain when the antenna is performing a transmitting function is the same as the gain when performing a receiving function. Many other antenna characteristics are also identical in both transmitting and receiving modes, but the descriptions are often couched only in terms of transmission. The illumination of an aperture may be thought of as the energy density distribution at the radiating opening (alternatively at the energy-collecting opening) of the antenna. In a horn antenna the radiating aperture is normally the large open end, corresponding to the open end of a trumpet. Thus, the illumination is the electromagnetic energy distribution within the opening of the horn.
As mentioned, the radiated beam shape depends upon the aperture illumination or distribution. It is well known that a relatively large aperture is capable of producing a relatively narrow radiated beam. Such a narrow radiated beam corresponds to an antenna having high directivity, and is ordinarily associated with high antenna gain. High gain or high directivity is a desirable characteristic of antennas used as feeds for reflectors. It is easy to understand that if an antenna has a large aperture, but most of the aperture is unused because the aperture illumination or distribution is such as to put little or no energy in a major portion of the aperture, that the useful or effective aperture is smaller than it would be if the illumination were uniform. For this reason, aperture illumination distributions which concentrate the energy in a small portion of the aperture, or in which the aperture distribution is other than uniform, result in a relatively wide radiation pattern, relatively low directivity and relatively low gain (although they may have other desirable properties such as low sidelobe levels). Such antennas may be less desirable for use as feed array elements for reflector-type antennas.